1. Field of the Invention
This invention relates to an anode assembly for rotating x-ray anode tubes, and in particular to an anode disc comprising a graphite substrate.
2. Description of the Prior Art
The longevity and efficiency of rotating x-ray anode tubes can be increased by using anode discs capable of high heat storing and high heat dissipating properties. Graphite possesses an exceptionally high thermal capacity when compared to molybdenum and tungsten, other materials used for making the substrate of the disc. At 1000.degree. C., the ratio of thermal capacity, in relative units, and in the order mentioned heretofore, is 48:7.4 and 48:4.1. The ratio of emissivity at 1000.degree. C. is 0.85:0.15 in both instances. However, the difficulty in using graphite as a substrate material is the problem of how to join the anode target to the graphite substrate.
Prior art anode assemblies embodying a graphite substrate suggest the use of zirconium hafnium as a suitable material for joining the anode target to the graphite substrate. However, both of these materials are carbide formers and present the problem of how to minimize the amount of carbide formed during the joining operation, as well as the desired working lifetime of the anode assembly, usually 10,000 x-ray exposures, minimum. The working lifetime subjects the anode assembly temperature to being cycled to reasonably high levels, the order of 1200.degree. C., and, therefore, continued carbide formation is a distinct possibility. The mechanical properties of a carbide layer formed in such an anode assembly may preclude the use of such an anode assembly in rotating x-ray anode tubes subjected to large amplitude thermal cycling.
Rhenium has been employed as a material for joining the anode target to the graphite substrate. Rhenium does not form a carbide at the temperature of joining or at the operating temperature of the tube assembly. However, the solubility of carbon in rhenium is relatively high and permits the diffusion of carbon therethrough and into the material comprising the anode target. Consequently, the material of the anode target may be embrittled by the formation of tungsten carbide. As a result, the operating lifetime and efficiency of such anode assembly designs are the same as, or less than, that of currently employed all-metallic anode assemblies.
It is therefore an object of this invention to provide a new and improved anode assembly for a rotating x-ray anode tube wherein the substrate of the disc is made of graphite.
Another object of this invention is to provide a suitable material for joining the anode target to a graphite substrate wherein the material is a non-carbide former and has a low solubility for carbon at the maximum, bulk operating temperature of a rotating x-ray anode tube.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.